The invention in this case relates to a novel semiconductor heterostructure and to a method of producing such a structure.
The invention also relates to a novel laser diode (LD) and to a novel light emitting diode (LED) capable of emitting radiation in the blue-green light emitting region employing the semiconductor structure of the invention.
Kukimoto U.K. Patent Application 2,204,731A shows a light emitting diode having an n-doped GAAs substrate and an n-doped layer of a II-VI compound deposited on the substrate and a p-doped layer of a II-VI compound deposited on the n-doped layer.
More particularly, this U.K. application shows a layer of a n-type ZnS.sub.x Se.sub.1-x deposited on a n-type crystal of GaAs and a layer of a p-type ZnS.sub.x Se.sub.1-x deposited on the surface of the layer of the n-type ZnSe.sub.x Se.sub.1-x and forming a pn junction therewith. Electric contact layers are provided on the surfaces of the layer of the p-type compound and the structure.
While the resulting structure has been found to function as a blue-light emitting diode it is of little or no commercial use as its efficiency (measured as a ratio of the amount of light produced to the electrical power applied) is very poor. One reason for this poor efficiency is considered to be the relatively high resistance present between the GaAs substrate and the layer of the ZnS.sub.x Se.sub.1-x deposited on the substrate. This is also considered to be on when, unlike the configuration shown in this U.K. application, the substrate and the layer deposited thereupon are p-type.
Suzuki JP 62-88329(A) shows a structure in which the diffusion of Ga from a GaAs substrate to a, layer of a p-type ZnSe compound is prevented by the use of an intervening layer of AlAs P or Al In P.
While the structure of Suzuki exhibits a relatively low resistance P-type conductivity the resistance is not so low as to enable the manufacture of commercially efficient blue-light emitting diodes and laser diodes.
Ogura JP 64-24980 shows a structure for a laser diode in which a first layer of n-type GaINP is provided on a n-type GaAs substrate, a second layer of n-type of AlGaInP is provided on the first layer, a third layer of a GaInP is provided on the second layer, a fourth layer of p-type AlGaInP is provided on the third layer and a layer of ZnSe or ZnSSe is provided so as to extend into and surround a portion of the fourth layer. A layer of p-type GaInP on which a layer of p-type GaAs is provided on the surface of the ZnSe or the ZnSSe layer. However, this diode exhibits red emission rather than blue emission.
M. Onomura et al, Electronics Letters Online No.; 19931402 shows a laser diode having a CdZnSe/ZnSe MQW (multiquantum well) structure in which a band offset reduction layer of p-type InAlGaP is provided between a p-type GaAs substrate and a cladding layer of p-type ZnSe.
The Onomura et al article indicates that the voltage drop between the layer of the p-type GaAs substrate and the p-type ZnSe layer due to the large Schottky and/or heterobarriers therebetween is considerable reduced by the presence of the intervening layer of the p-type InAlGaP layer and indicates that a laser diode comprising such a structure is operable at a temperature of 77K with fairly good efficiency. However, there is no indication in this article of the laser diode operating at a sufficient efficiency at room temperatures so as to make it commercially useful.
Ogura et al JP 89-24980 shows a laser diode comprising a GaAs substrate, a GaInP or AlGAInP active layer provided between two cladding layers having a wider energy band gap than the active layer, the cladding layers containing AlGaInP or AlInP and the cladding layer remote from the substrate having a mesa stripe shaped buried in a ZnSSe layer lattice matched with the active layer or one of the cladding layers.
This laser diode exhibits red light emission rather than blue light emission.